Direct contact water heaters are known, such as described in U.S. Pat. No. 4,574,775 issued on Mar. 11, 1986 and comprised of a vertically oriented cylindrical column having a packing adjacent an upper end thereof. Water to be heated is sprayed on top of the packing so that the water is heated by the packing and also by hot gases passing through the cylindrical column. The hot gases are usually provided by a fossil fuel burner which is installed at the bottom of the column to produce hot flue gases which are directed upwardly in counter-current flow to water droplets falling from the packing. The energy of the flue gas is absorbed by the down-coming water droplets and these droplets are further heated when entering into direct contact with the flame. Hot water is stored at the bottom of the column from where it is pumped to supply external devices.
Direct contact flue gas stack economizers operate substantially as direct contact water heaters with the exception that the hot flue gases are generated from other sources. Flue gases from those other sources are admitted into the column below the packing and the energy of the flue gases is absorbed by the down-coming water. Although these stack economizers are considered to be an efficient way of recovering lost heat, they have two main disadvantages, one being that the maximum outlet temperature of the column is approximately the dew point temperature of the flue gases entering the column. Also, the maximum amount of energy which can be transferred to the water depends, and is limited by, the actual flue gas flow and temperature available from existing external apparatus, and this can vary at different time intervals. Therefore, an additional amount of heat may be needed in order to supplement the recovered heat to meet the process demand. This additional amount of heat may be added by a direct contact water heater or by other means. It must be kept in mind, however, that the disadvantages of direct contact stack economizers are compensated by the big advantage of free energy from the existing flue gas recovered from other sources which was previously lost to the atmosphere.
The direct contact water heaters do not have the same outlet water temperature limitation as does the direct contact stack economizer, and can heat water well above the dew point of the combustion gases. Also, the direct contact water heater can be sized for any amount of energy required, as it has its own burner. However, one of the disadvantages of the conventional direct contact water heater is that it heats water at a very high efficiency level with fossil fuel, but this fossil fuel is costly as compared to free energy being recovered by direct contact economizers. Also, the temperature of the flue gases being exhausted by the direct contact water heater is equal or slightly higher than the incoming water. In the case of water being preheated by a direct contact stack economizer, and where a direct contact water heater would be subsequently utilized to add the additional energy required, the flue gases exhausting from the heater would still be hot enough that it would be economical to channel them into the direct contact stack economizer for further cooling down of the exhaust gases. Accordingly, there is a waste of energy.
There is therefore a need to provide an ideal water heater arrangement wherein cold water is introduced at the top of a direct contact stack economizer and is preheated by hot flue gases. Preheated water at the bottom of the direct contact stack economizer would then be transferred to the top of a direct contact water heater to be further heated. Flue gas exhausting from the direct contact water heater would then be directed to the direct contact stack economizer to be cooled down as much as possible. These flue gases would combine with flue gases of other apparatus. However, the main disadvantage of this arrangement would be the cost of fabricating and interconnecting two separate pieces of equipment, namely a direct contact stack economizer and a direct contact water heater.